This invention relates to a process for the removal of arsenic, tin and antimony from crude lead containing silver by means of industrial oxygen in a lead smelting crucible, and a device for the execution of the process.
The removal of tin, arsenic and antimony from crude lead containing silver is presently carried out in lead matallurgy in accordance with either the Harris process or the reverberatory process.
The Harris process (Ullmann, 3rd edition, volume 4, pages 498-501) is used, along with the separation of tin, arsenic and antimony, for the processing of tin-rich and/or tellurium-rich lead, whereby valuable final products accumulate which are in part highly concentrateo. The separation of the above-stated impurities from the crude lead takes place with sodium hydroxide and a strong oxidizing agent, preferably saltpeter, forming Na.sub.3 SbO.sub.4, Na.sub.3 AsO.sub.4 and Na.sub.2 SnO.sub.3 which accumulate in the form of a fluid salt slag. The impurities removed from the crude lead subsequently must be separated from the salt slag into concentrateo and lead-free products by means of hydrometallurgic processes. The processing of the salt slags, which is the actual nucleus of the Harris process, requires extensive equipment and thus correspondingly high installation costs. The process is also expensive and requires careful monitoring. For these reasons the Harris process has not yet become widely prevalent in most lead smelting plants.
In the more widely used reverberatory furnace process (Ullmann, 3rd edition, volume 4, pages 498-501), the antimony along with the arsenic and tin, is oxidized at 700-750.degree. C. by means of atmospheric oxygen. For this purpose, rectangular reverberatory or refining furnaces are used and the exhaust gasses, after the temperature has dropped, are conveyed in a cooling unit to a filter for removing dust. The air, which is blown in through lance units into the lead bath, oxidizes the tin, arsenic and antimony in that sequence forming double oxides which are removed from the furnace as fluid dross. Depending on whether a continuous or discontinuous operation is used, drosses of 8-25% Sb, 1-5% As and 30-50 ppm Ag are produced. In a continuous reverberatory furnace process which is characterized by its high conversion rate, drosses with only 8-13% Sb result. The low antimony content leads to correspondingly high dross quantities resulting in increased processing costs. The drosses are additionally processed by means of reduction melting into an alloy containing antimony and lead which is termed " crude hard lead", and from which hard lead qualities of commercially conventional quality are produced by means of subsequent refining.
The above-stated processes are characterized by high equipment expenses, such as for example, the processing of salt slags in the Harris process, and the cooling unit, dust removal filter, reserve furnaces and soon in the reverberatory furnace process. They are also characterized by a high energy consumption for the processing of large quantities of intermediate products, such as salt slags and drosses, as well as by high operating costs.
Modern processes in both primary as well as secondary lead smelting works for the separation of tin, arsenic and antimony from crude lead use oxygen/air mixtures in a conventional lead smelting crucible. In secondary lead smelting plants the drosses can be processed without problem because the crude lead of the secondary lead smelting plants has only a very low silver content (&lt;30 g Ag/t). In a primary lead smelting plant in which a crude lead containing a silver content of up to several thousand g Ag/t is similarly processed, a dross is produced with 3.85% arsenic, 3.25% antimony and 1098 g/t of silver. See Proceedings of the CIM Symposium on "Quality in Non-Ferrous Pyrometalllurgical Processes", Vancouver [1985], pages 137-140. During the subsequent reduction of the dross, the silver contained therein moves into the hard lead from which it can not be removed, resulting in a corresponding loss of valuable metal. Furthermore, the hard lead which contains silver can not be marketed as commercial hard lead because the silver exceeds the permitted limits. Thus, this method can only be carried out in primary lead smelters which process crude lead containing silver if, before the reduction to crude hard lead, the dross is separated in a separate process step by liquation into an Ag-poor dross and Ag-rich crude lead. The liquation process is carried out for example in a short-drum rotary furnace or in a liquation hearth furnace. Because of the additional expense of liquation, the advantage of the crucible refining is considerably reduced.
Thus, it is clear that crucible refining by means of air enriched with oxygen can only be carried out economically, if the silver contents in the crude lead are very low, such as for example during the refining of scrap lead in accordance with DE-PS 3 332 796 which must be operated at temperatures of at least 630.degree. C.
The task of the present invention is to describe a device and a process which avoid the above-stated disadvantages, such as for example crosses containing silver, additional consumption of reagents, higher operating temperature, etc., during refining in the lead smelting crucible, and in which the removal of tin, arsenic and antimony is carried out through the use of industrial oxygen in a conventional lead smelting crucible.
This task is solved in the process of the type described above, by introducing oxygen into a turbulent stream of fluid lead which is constricted to a proportionate volume relative to the smelting crucible. With such introduction the lead, thoroughly mixed with oxygen, enters into a larger volume for the purpose of calming, and the elements which are to be separated off float on the surface in the form of oxides and are skimmed off.
The process is carried out in a device which comprises two cylinders of different volumes. The cylinders are positioned perpendicularly to one another, can be adjusted relative to one another, and they project out above the surface of the molten mass. The cylinders are suspended on a traverse unit and the entire smelting crucible is covered by a protective hood. The turbulent stream of the lead is produced by means of lead pump, the discharge opening of the pressure side of which lies above the level of the lead. The turbulence in the small cylinder can also be produced below the level of the lead by means of a nozzle shaped discharge from the pump.